Update: Ubisoft has issued a statement about Rayman Legends, acknowledging its development but stressing it's for internal use only:

An internal video showing images of Rayman Origins’ sequel has leaked over the Internet. This video was intended as a purely internal demonstrative video, and in NO way represents the final game, the final console or their features. This video was destined for internal production teams who often create game prototypes with work in progress development kits. Ubisoft confirms the development of Rayman Legends handled by Michel Ancel and his team in Montpellier.

The site grabbed a video of Rayman Legends, a new four-player co-operative platformer that promises exclusive content for Wii U. The video shows multiplayer with the Wii U controller being used to move platforms and display gameplay.